Header

Every time VTAM® sends a message to a program operator or every time a program operator sends a VTAM operator command to VTAM, a 4-byte header is appended to the data. The header has the following form:

X'00'	Status	ID number
1 byte	1 byte	2 bytes

X'00': Is a required part of the header and is always present. Either it is provided by VTAM in data that the program operator receives, or it must be supplied by the program operator in data that is sent to VTAM.
Status: Indicates the sender of the message or command, whether a reply is required, and the type of message if it is part of a multi-line message.
When VTAM sends an operator command to the program operator, the program operator sets the following bits:

Status field bit setting 0123 4567	Meaning
`.... ...1`	The command originates from a program operator.
`.... ...0`	The command originates from VTAM
`.... ..0.`	VTAM is not to return a reply to the message issued as a result of a SENDCMD macroinstruction.
`.... ..1.`	VTAM returns an appropriate reply to the message issued as a result of a SENDCMD macroinstruction.
`xxxx xx..`	Reserved.

When the program operator receives a message from VTAM, VTAM sets the following bits:

Status field bit setting 0123 4567	Meaning
`.... ...0`	This message is an unsolicited message. It originates from VTAM and is not in reply to a VTAM operator command. The ID in this header was generated by VTAM.
`.... ...1`	This message is sent in reply to a VTAM operator command previously sent to VTAM by the program operator. The ID number in this header was generated by the program operator when the command was sent.
`.... ..0.`	A reply is not required.
`.... ..1.`	A reply is required. A reply ID is present in the message. Use the SENDCMD macroinstruction to issue a REPLY command to VTAM.
`.... 00..`	This message is not a copy of a VTAM operator command or a copy of a VTAM message sent to the system console.
`.... 01..`	This is an unsolicited message containing a copy of a VTAM operator command entered from the system console.
`.... 10..`	This is an unsolicited message containing an unsuppressed message that had been sent to the system console.
`.... 11..`	This is an unsolicited message containing a copy of a VTAM-solicited, suppressed message that had been sent to the system console.
`0000 ....`	This is not a multi-line message.
`0001 ....`	This is the *control line* of a multi-line message. The control line is always the first line of a multi-line message and normally contains a message title. The control line should remain static during framing operations on a display console such as the 3270 (provided it is displayed in an out-of-line display area).
`0010 ....`	This is a *label line* of a multi-line message. The label line might be the first line of a multi-line message if there is no control line. If there is a control line, the label line always follows it. There can be more than one label line, but they cannot be interspersed with other types of lines. Label lines usually contain message header information and remain static with the control line.
`0100 ....`	This is a *data line* of a multi-line message. The data line always follows either a control line or a label line, if present. The data line contains the information intended for the VTAM operator and, unlike the control line and label line, is paged during framing operations.
`1000 ....`	This is the *end line* of a multi-line message. The end line indicates that the previous data line is the last line of text that is to be passed to the VTAM program operator. If data is included in the end line, it is ignored.
`1100 ....`	This is a combined data and end line of a multi-line message. It indicates that this is the last line of text that is passed to the program operator.

ID number: Any number 0–65535 (X'0000'–X'FFFF') can be specified by VTAM when it sends an unsolicited message to the program operator. The program operator, when sending a command or reply to VTAM, can select and interpret the identification number in any way it finds meaningful. When the program operator sends a command to VTAM, the program operator must set the identification number; if a reply is requested, VTAM returns the same number with the reply. If VTAM sends a message to the program operator and the message is not a reply to a command, VTAM sets the identification number. Using the identification number, the program operator can correlate solicited messages received from VTAM with the appropriate commands issued by the program operator.

Note: An IBM-supplied DSECT (ISTDPOHD) is available and can be used in creating or interpreting the header. Refer to Format and DSECT of the message and command header for more information.